This invention relates to small electronic apparatus containing an integrated circuit (IC) and a secondary cell and more particularly to an IC card with an improved charging system for a secondary cell contained therein.
IC cards with functions both as an embossed card and as a magnetic card are coming to be widely used. Such an IC card generally has on its front surface a magnetic stripe for writing data thereon and reading data therefrom so as to function as a magnetic card, an embossed section where certain data necessary for the card are imprinted, and a plurality of contact means for connection to an external apparatus. Some IC cards are further provided on the back surface with a display device for displaying data, etc., a key input section with a plurality of keys used for controlling the display and a power source such as a cell.
With an IC card thus structured, it is not easy to replace the cell with a new one because the card is of a limited thickness. Use is therefore commonly made of a paper-like 3 V secondary cell of lithium. Since an organic electrolyte is generally used in such a secondary cell, the electrolyte may be decomposed if a voltage greater than 3 V is applied for recharging. If this happens, the cell can no longer serve as a rechargeable cell.
In view of this problem, there have been IC cards with a circuit as shown in FIG. 2 for charging the secondary cell. With reference to FIG. 2, the circuit is comprised of a power source 2, an IC 4 driven by the power from the source 2, a control key section 6, a display device 8 for displaying data, etc. in response to display signals from the IC 4 and a group of eight pin terminals (collectively indicated at 10) for the IC 4. The group of these pin terminals 10 is for connecting the IC card to a terminal device and is comprised of free pin terminals 10a and 10b, an input/output (I/O) pin terminal 10c, a pin terminal 10d for resetting, a pin terminal 10e for a clock signal, a pin terminal 10f for receiving power, for example, from a 21 V first power source (V.sub.pp), a pin terminal 10g for receiving power, for example, from a 5 V second power source (V.sub.cc), and a grounding pin terminal 10h. Symbol 10i indicates a terminal for recharging to be described below.
The power source 2 is comprised of solar cells 12, a reverse current preventing diode 14 connected in series with the solar cells 12 with polarity backward with respect to that of the solar cells 12, a secondary cell 18 which is connected in parallel to the series connection 16 of the solar cells 12 and the diode 14, and another reverse current preventing diode 20 serving to prevent the secondary cell 18 from becoming charged directly by a voltage applied to the pin terminal 10g for receiving power. The solar cells 12 and the secondary cell 18 are so connected that their terminals of the same polarity are joined together.
With an IC card thus structured, the power source 2 is usually used as the source of power for the IC 4. The secondary cell 18 inside the power source 2 is charged by the solar cells 12 through the reverse current preventing diode 14. When there is no incident sun light, causing the voltage of the solar cells 12 to drop, and when the secondary cell 18 remains uncharged for a long period of time, the voltage from the secondary cell 18 may drop to such an extent that the IC 4 cannot be properly driven any more. In such a situation, the user may wait until there is sunshine again and the voltage of the solar cells 12 rises. If it is desired to recharge the secondary cell 18 immediately so as to make the IC 4 operable, however, an external recharging device must be used to recharge the secondary cell 18. This is done by connecting corresponding terminals 24a and 24b of a dedicated recharger 22 respectively to the recharging pin terminal 10i and to the grounding pin terminal 10h and by quickly and directly charging the secondary cell 18 by the recharger 22.
In summary, a conventional IC card thus operated must be provided with a recharging terminal 10i in addition to the usual set of eight pin terminals 10a-10h for the regular functions of an IC card. In other words, this extra terminal 10i required for the recharging increases the production cost of the card. Moreover, an extra space must be provided for disposing such an extra terminal and this adds to the design restrictions of the card. A further disadvantage of IC cards of this type is that a dedicated recharger for the IC card must be provided for recharging the secondary cell. This also has the adverse effect of increasing the operating cost of the IC card.
These problems of the conventional technologies arise not only in connection with IC cards but also in connection with all kinds of thin electronic apparatus such as electronic notebooks and electronic desk calculators containing ICs and secondary cells.